Field of the Invention
The present invention relates to a combustion process and an apparatus therefor containing separate fuel and oxidant injectors to introduce fuel gas and oxidant separately in the combustion chamber of a furnace, so that the fuel burns with the oxidant in a wide luminous flame, and whereby the combustion of the fuel with the oxidant generates reduced quantities of nitrogen oxides (NOx).
Related Art
Industrial high temperature processes, such as glass or frit melting, ferrous and non ferrous materials smelting, use large amounts of energy to transform a variety of raw materials into a hot product that is then cast, formed or otherwise disposed of in further stages of the industrial process. This operation is generally performed in large furnaces that can produce as much as 500 tons (metric) per day of molten material. Combustion in the furnace of high calorific value fuels, such as natural gas, propane, or possibly low calorific fuels such as some blast furnace gasses, with an oxidant that contains oxygen is a preferred method of supplying the energy. In some cases, the combustion is supplemented by electric heating. Most of the time, fuel and oxidant are introduced in the furnace through burners in order to generate flames. The transfer of energy from the flames to the material to be heated or melted results from the combination of convection at the surface of the material and radiation to the surface or into the material, if the material is transparent to the radiation. Flames that are highly radiative (usually referred to as luminous flames) are usually preferred, because they provide better heat transfer and, thus, higher fuel efficiency.
For flame heating, it is also very important to have the energy from the flame evenly distributed above the surface of the material to be heated or melted. Otherwise, hot and cold regions may co-exist in the furnace, which is generally not desirable as it may affect the quality of products manufactured with material heated or melted in such a furnace. For example, in a bath of molten glass, there may be glass stones in cold regions, and increased volatilization in hot regions. Wide flames are preferred because they yield a better and more even coverage.
In many countries, increasingly stringent regulations are being promulgated regarding emissions of NOx. Combustion techniques wherein NOx formation is limited have therefore been developed.
In very high temperature processes, NOx formation is promoted by long residence times of oxygen and nitrogen molecules in hot regions of the flame and the furnace. The use of substantially pure oxygen (90% O2 or higher) instead of air as the oxidant has proven to be very successful in reducing the NOx emissions by as much as 90%, since nearly all nitrogen is eliminated.
However, substitution of air by substantially pure oxygen increases the flame temperature, and thus creates regions in the furnace where the reactivity of nitrogen with oxygen is high, and wherein the formation of NOx may proportionally increase, even though it is globally decreased when compared to combustion with air. Indeed, it is, in practice, usually impossible to eliminate all nitrogen from a furnace, because industrial furnaces are not tight to air leaks, the fuel usually contains some nitrogen, and oxygen supplied from non-cryogenic sources, such as oxygen produced by a Vacuum Swing Adsorption plant (VSA), contains a small residual nitrogen concentration.
Burner assemblies capable of operating at low pressure, particularly for the oxidant gas, while producing a wide, flat luminous flame with reduced NOx emissions, and which afford a manner of controlling flame length so as to adapt the flame to the furnace in which they are used are known from U.S. Pat. Nos. 5,984,667 and 6,068,468, and are commercialized by the applicant under the commercial denominations Alglass™ FC and Albatch™ FC.
An essential aspect of said burner assembly is that the fuel and oxidant are physically separated (i.e. spaced apart), and geometrically arranged in order to impart to the fuel fluid streams and the oxidant fluid streams angles that allow combustion of the fuel fluid with the oxidant in a stable, wide, and luminous flame.
More specifically, U.S. Pat. No. 5,984,667 discloses a burner assembly comprising at least two fuel fluid cavities, at least one oxidant fluid cavity and at least one exit face at which at least one of the fuel fluid cavities and at least one of the oxidant fluid cavities terminates. Said burner assembly comprises:    a) means for supplying an oxidant fluid stream;    b) means to inject said oxidant fluid stream in said at least one oxidant fluid cavity to create at least one injected oxidant fluid stream;    c) means for supplying a fuel fluid stream; and    d) means to inject said fuel fluid stream in said at least two fuel fluid channels to create at least two injected fuel fluid streams;    whereby the directions of injection of the oxidant fluid stream and the fuel fluid stream are substantially converging and intersect at a combustion zone, while the directions of at least two adjacent fuel fluid channels are diverging.
Demonstrated benefits of this type of burners are:                improvements in fuel efficiency by directing the flame energy towards the charge rather than towards the furnace structure,        improvements in heating uniformity and elimination of hot spots through enhanced charge coverage,        high luminosity resulting in efficient heat transfer to the charge, e.g. in glass melting furnaces,        low pollutant emissions.        
Burners of this type operate reliably under a certain range of process conditions, in particular within a certain range of firing rates, but problems may arise when the burner is operated outside said range. For example, a problem experienced with burners of this type is that at low firing rates (e.g. below 30% of the nominal firing rate) the flame produced by the burner is ‘lazy’ and tends to lift towards the crown of the furnace, thereby increasing the risk of hot spot formation in the furnace crown. At very high firing rates (e.g. more than 200% of the nominal firing rate, flame control becomes increasingly difficult and the length of the flame may be such as to cause damage to the opposite furnace wall.
There exists a need to increase the flexibility of the above known type of burner.
In glass-melting furnaces, by way of example, the pull rate of the furnace may have to be increased, which generally entails an increase in the firing rate of at least one of the burners mounted in the furnace. Under certain circumstances, it may also be necessary to decrease the pull rate of the furnace, which may entail a decrease in the firing rate of at least of its burners. It would be desirable to provide more flexibility in the firing rate of the furnace without deterioration of the quality of the product produced in the furnace and without substantially accelerating or increasing the risk of furnace damage.
It is an object of the present invention to provide burner assemblies and methods of combustion which reliably present the benefits of the above known type of burners over a wider range of process conditions and in particular of firing rates.
Burners jointly injecting fuel and oxidant (i.e. fuel and oxidant injection which is not spaced apart as opposed to separate injection) and capable of operating at both high and low momentum are known in the art.
EP-A-0563793 discloses a variable-momentum oxy-fuel combustion system comprising an oxy-fuel burner having a generally cylindrical housing with a fuel conduit disposed in spaced relation to and concentrically within said housing, whereby:                the fuel conduit is co-extensive along a major portion of said housing and has a flame end terminating in the same plane as the flame end of said housing;        a fuel cap is disposed concentrically within the fuel conduit, said fuel cap and said fuel conduit cooperating to produce an annular fuel flow at said flame end of said housing;        an oxidizer conduit is disposed concentrically within said housing between said fuel conduit and said housing, said oxidizer conduit extending coexistively in said housing;        a precombustor is mounted on said burner;and whereby:        the fuel conduit is adapted for variable positioning relative to said oxidizer conduit along the longitudinal axis including a position where they terminate in the place defined by the flame end of said housing to define an annular oxidizer passage orifice surrounding said fuel conduit; and        the fuel conduit and said fuel cap adapted for variable positioning relative to each other along the longitudinal axis including a position where they terminate in the place defined by the flame end of said housing to define an annular fuel passage means to introduce fuel to said fuel passage and oxidizer to said oxidizing passage.        
A major inconvenience of the variable-momentum combustion system according to EP-A-0563793 is that the momentum of the oxy-fuel flame is varied by varying the position of constituent parts of the assembly, such as the fuel conduit, the oxidizer conduit and/or the fuel cap proximate the flame end of the housing. The presence, in the assembly, of moveable parts at a point where they are subjected to the effects of the oxy-fuel flame, such as high temperatures and possibly deposits of combustion products or volatilized material, affects the reliability of said combustion system in operation.
EP-A-763692 discloses an oxy-fuel burner including an outer oxidant tube, an intermediate fuel tube and an inner oxidant tube which are coaxially arranged with the fuel tube disposed between the inner and outer oxidant tubes and whereby the characteristics of the flame produced by the burner may be controlled by varying the relative flow rates of the inner and outer oxidant flows. An increase in the percentage of the total oxidant which is provided to the inner oxidant tube causes the length and luminosity of the flame to decrease and the flame momentum to increase. Burners of this type are commercialized by the applicant under the commercial denominations Alglass™ VM and Albatch™ VM.
Benefits of this burner technology as demonstrated in glass-melting furnaces are:                possibility to regulate the atmosphere in the furnace,        possibility to regulate the momentum of the flame, in particular in the case of oxy-boosting,        possibility to adjust the flame length according to furnace geometry (furnace width in the case of cross-firing), and        possibility to modify the flame luminosity according to the type of charge (glass composition).        
Neither of the above known variable momentum combustion technologies for joint injection of fuel and oxidant are adapted for use in burner assemblies of the type known from U.S. Pat. Nos. 5,984,667 and 6,068,468, which comprise at least two fuel passages and whereby fuel and oxidant gas are injected in separate fluid streams into a combustion chamber of a high temperature furnace.